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8.1
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Stars
Here is a summary of what you
will learn in this section:
• A star is a huge ball of hot gas,
or plasma. Nuclear reactions in
its core turn matter into energy.
• A star forms inside a nebula as
gravity pulls dust and gas
together, creating a spinning,
contracting disk of material in
which nuclear fusion begins.
• Stars have life cycles during
which they form and then evolve
in one of three main ways.
• Eventually, most stars cool
down and slowly grow cold and
dark. Some, however, expand
into giants before then cooling
down slowly or exploding as a
supernova.
Figure 8.1 The Big Dipper seen at dusk over Lake Ontario
Stars: The View from Earth
W O R D S M AT T E R
“Constellation” is derived from the
Latin words con, meaning with or
together, and stella, meaning stars.
“Asterism” is from the Greek word
aster, meaning star.
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If you have ever spent time looking up at the sky on a clear night,
you have probably noticed that some stars look as though they are
grouped together into a distinct pattern. Perhaps the best-known
star pattern in the northern hemisphere is the Big Dipper (Figure
8.1). Different cultures around the world refer to this collection
of seven stars by other names, such as the Plough, the Ladle, and
the Great Cart.
The Big Dipper is part of a larger star pattern known as Ursa
Major, which is Latin for Great Bear (Figure 8.2). Ursa Major is
an example of a constellation. A constellation is a group of stars
that, from Earth, resembles a recognizable form. Astronomers
have officially listed a total of 88 constellations. Examples, along
with Ursa Major, include Cassiopeia, Orion, Pegasus, Sagittarius,
and Ursa Minor. Smaller recognizable star patterns within a
larger constellation are known as asterisms. The Big Dipper is
an asterism. Star patterns like these are just one kind of
astronomical phenomenon, a term that refers to any observable
occurrence relating to astronomy.
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It is easy to think that all the stars forming a constellation
or asterism lie at the same distance from Earth, as though
drawn on the ceiling in
your classroom. In fact,
the stars in the pattern
vary greatly in their
distances from Earth,
with some being many
times farther away than
the others. They only
appear to be twinkling
from a flat surface
because they are of
similar brightness.
Suggested Activity •
C12 Inquiry Activity on page 302
Figure 8.2 The constellation Ursa
Major. To many cultures, this star
pattern looked like a large bear.
C11 Quick Lab
Reading Star Charts
Star charts are maps that show some or all of the 88
constellations and key stars that are visible from
Earth.
Stargazers use star charts to orient themselves to
the night sky, just like people use maps to find their
way around new places on the ground. If someone
told you about an interesting star cluster in the
constellation of Aquarius, for example, knowing where
to look on a star chart would allow you to see the star
cluster too.
Purpose
To use a star chart to determine the location and
appearance of well-known stars, constellations, and
asterisms visible in the the northern hemisphere
Procedure
1. Working on your own, turn to the star chart in
Skills Reference 12 or use the handout that your
teacher gives you.
Questions
2. Looking at the star chart, answer the following
questions.
(a) In which constellation is Polaris (the North
Star) located?
(b) What planet is shown in the constellation
Capricornus?
(c) Betelgeuse is a large star located in what
constellation?
(d) What is the name of the constellation that has
three bright stars in a row?
(e) What is the name of the star that seems to
form the tail of the swan-shaped constellation
known as Cygnus?
(f) Is the star Aldebaran located east or west of
Betelgeuse?
(g) What is the name of the star cluster located
midway between the constellations of Taurus
and Perseus?
(h) What large star seems to form the right foot of
the constellation commonly referred to as
Orion the Hunter?
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How a Star Is Born
Compared with the life span of humans, the life span of stars is
extremely long. All stars form inside a collapsing nebula, a cloud
of dust and gases. A nebula’s collapse can be triggered by a
disturbance such as the gravitational attraction of a nearby star or
the shockwave from an exploding star.
Inside a collapsing nebula, the region with the greatest
amount of matter will start to draw material towards it through
gravity. This is where the star will form (Figure 8.4a). Material
falling inward to the core has excess energy. This energy causes
the central ball of material to begin to spin (Figure 8.4b).
Extremely high pressures build up inside the ball, which in turn
causes the tightly packed atoms to heat up. As the temperature
climbs, the core begins to glow. This is a protostar (Figure 8.4c).
A protostar is a star in its first stage of formation.
Eventually, the temperature of the spinning protostar rises to
millions of degrees Celsius. This is hot enough for nuclear
reactions to start. Over tens of thousands of years, the energy
from the core gradually reaches the star’s outside. When that
occurs, the fully formed star “switches on” and begins to shine.
(a)
(b)
The Life Cycle of Stars
A century ago, astronomers could tell that different kinds of stars
existed. What they had not yet discovered was that stars have a
predictable life cycle just like all living things do. It took the work
of two researchers in the early 1900s to find the key to
understanding star evolution.
(c)
Figure 8.4 (a) As a region of a
nebula collapses in on itself, gravity
starts pulling dust and gas together
into small masses. (b) As a mass
grows, it begins a cycle of heating
up, spinning, contracting (pulling
inward), more heating, and so on.
(c) The result of this process is a
protostar.
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Figure 8.3 Stars are “born” in nebulae, such as the Eagle Nebula shown here, with its aptly
named star-forming “Pillars of Creation” region (inset).
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Star Mass and Evolution
During Reading
How a star evolves in its lifetime depends on the mass it had
when it originally formed. Astronomers describe stars in three
general mass categories: low, medium, and high. A low mass star,
for example, advances through different phases than a high mass
star does.
Low Mass Stars
Low mass stars use their nuclear fuel much more slowly than
more massive stars do. Low mass stars burn so slowly that they
can last for 100 billion years — more than eight times the current
life span of the universe.
With less gravity and lower pressures than other stars, the
nuclear reactions in the core of low mass stars happen at a
relatively slow rate. The stars therefore exist a long time, shining
weakly as small red stars called red dwarfs (Figure 8.5).
Like the light from a flashlight whose batteries are almost
dead, the light of a red dwarf starts dim and gradually grows
dimmer. As red dwarf stars run out of fuel, they collapse under
their own gravity. This causes the star to reheat, but not enough
that nuclear fusion can begin again. Most of the stars in the
universe are red dwarfs.
Red dwarf stars eventually cool into smaller white dwarfs.
As you read about the evolution
of stars, create a chart to
compare the different types.
Note the types of stars, their
names, examples, and two
important facts about each type.
Which type of star has the
longest life? Which type always
comes to a violent end?
red dwarf
white dwarf
black
dwarf
Comparing Important Ideas
nebula
white dwarf
protostars
black hole
red
giant
main sequence
star = 1 solar mass
neutron
star
supernova
supergiant
massive main
sequence star =
100 solar masses
Figure 8.5 The three main life cycles of stars. What cycle a star goes through is
determined by what mass the star first develops after its formation in a nebula.
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Medium Mass Stars
Medium mass stars burn their fuel faster than low mass stars do,
using their hydrogen up in about 10 billion years. The Sun is a
medium mass dwarf star.
At the end of this long, stable period, the hydrogen fuel in a
medium mass star begins to run out and the star slowly collapses
under its own gravity. This process of collapsing raises the
temperature and pressure again inside the star. This is enough to
start the fusion of helium, which has been accumulating in the
core. The star reignites. As the core heats up this time, the star
expands rapidly into a red giant (Figure 8.5). Aldebaran, for
example, is a red giant.
Eventually, even the helium fuel burns out and the star
collapses again and slowly burns out.
Figure 8.6 Polaris, the North Star, is
a supergiant. It is more massive
than the Sun and 1000 times
brighter. Unlike the Sun, however,
Polaris is very unstable.
High Mass Stars
High mass stars are those that are more than 10 times the mass of
the Sun. In a high mass star, as gravity pulls matter into the
centre of the star and squeezes the core, the nuclear reactions
accelerate. As a result, a high mass star is hotter, brighter, and
bluer than other stars (Figure 8.5).
High mass stars always come to a violent end. After using up its
hydrogen fuel, typically in less than about 7 billion years, such a star
collapses just like a low or medium mass star does. The heating and
compression cause helium to begin to fuse. During this process,
tremendously high temperatures result, causing the star to expand
into a supergiant. Examples of supergiants are Polaris (Figure 8.6)
and Betelgeuse (Figure 8.7).
When the helium fuel runs out, the core again collapses into
itself. The star continues to go through many cycles of collapse and
expansion, as new elements, including iron, are formed in its core.
Figure 8.7 Betelgeuse is
a red supergiant. It is so
huge that if it were in
the solar system where
the Sun is, it would
reach nearly all the way
to Jupiter’s orbit.
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Supernovas: The Violent End of High Mass Stars
When iron fuses, it does not do so in a way that releases energy. If
too much of the core of a high mass star is made up of iron, the
star — which may have been shining continuously for more than
7 billion years — will “turn off” in minutes. With no fuel left to
keep it producing heat energy, the star collapses one final time. So
fast and intense is the collapse that the core of the star heats up to
many hundreds of millions of degrees and explodes. As noted in
section 7.1, an exploding star is called a supernova.
The explosion releases enough energy to cause the iron and
other elements to fuse in various combinations (Figure 8.8). In
this way, all the elements of the periodic table have been formed.
The blast sends these heavy elements far out into space. Some
of the debris and elements from the old star create new nebulae
out of which new star-and-planet systems may begin to form.
The star’s remaining core after a supernova explosion faces
one of two outcomes, depending on the mass of the original star:
Figure 8.8 Are you wearing jewellery
that contains silver or gold, or do
you have a copper penny in your
pocket? The atoms in all heavy
elements were produced in a
supernova.
• Neutron stars — If the star was between 10 and 40 times the
mass of the Sun, it will become a neutron star. A supernova
explosion is directed not only outward, but also inward.
This force causes the atoms in the star’s core to compress
and collapse. When an atom collapses, it forms neutrons,
particles that are at the centre of most atoms already.
When the star’s core becomes little
more than a ball of neutrons only about
15 km across, it is called a neutron star.
Neutron stars are made of the densest
material known (Figure 8.9).
• Black holes — If the star was more than
40 times the mass of the Sun, it will
become a black hole. After exploding as
a supernova, the star’s core is under so
much gravitational force that nothing
can stop its collapse, not even the
formation of neutrons. In this case, the
effect of gravity is so great that space,
time, light, and other matter all start to
fall into a single point.
As noted in section 7.1, black holes
grow with the more mass they pull in.
Figure 8.9 A neutron star. Imagine the dome at the Rogers Centre
in Toronto being filled to the brim with steel and then that amount
of steel being compressed to fit inside a 20-L fish tank. That is how
dense the matter is in a typical neutron star.
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The Hertzsprung-Russell Diagram
Suggested Activity •
C13 Design a Lab on page 303
C14 Quick Lab on page 304
As the life cycle of stars shows, stars occur in many varieties. The
differences between them include what colour they are, how
bright (or luminous) they are, and even what their surface
temperature is (Figure 8.10).
In 1919, two astronomers decided to sort and plot thousands
of stars according to these three characteristics. Ejnar
Hertzsprung and Henry Norris Russell wanted to find out
whether any patterns might emerge that would tell us more about
the nature of stars. The results of this survey and plotting work
became one of the most important discoveries in astronomy in the
20th century.
The plotted data revealed for the first time that very clear
relationships existed between star properties. Figure 8.11 shows a
version of what is called the Hertzsprung-Russell diagram. In it,
the stars are arranged as follows:
• by colour – Red stars are plotted on the right, and blue stars
are plotted on the left. Other stars, such as the yellow Sun,
are plotted in between.
• by luminosity – The brightest stars are plotted at the top,
and the dimmest stars are plotted at the bottom. A star with
a luminosity of 100 is 100 times brighter than the Sun.
• by surface temperature – The hottest stars are plotted on
the left, and the coolest stars are plotted on the right.
Figure 8.10 The stars shown in this
binary system differ in colour,
luminosity (brightness) and surface
temperature.
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Deneb
Betelgeuse
supergiants
Rigel
Polaris
10 000
Antares
Arcturus
red giants
Procyon A
Vega
Luminosity (Sun = 1)
100
Spica
m
a
1
Aldebaran
i n
Sirius A
s e
Sun
q u
0.01
e
Sirius B
white
dwarf
0.000 1
n
Procyon B
c
e
Proxima Centauri
0.000 001
15 000
12 000
9 000
6 000
3 000
Surface Temperature of the Star (°C)
Figure 8.11 The Hertzsprung-Russell diagram represents the plots of thousands of stars based on colour,
luminosity (brightness), and surface temperature.
The Hertzsprung-Russell diagram shows many patterns based
on the three star properties noted above. For example, the star
data forms a distinct band that stretches from the top left of the
diagram to the bottom right. This is called the main sequence.
The Sun is a main sequence star. These stars are thought to be in
the stable main part of their life cycle. They have evolved to this
stage since formation but will gradually either cool and die out or
expand before exploding.
Groups of stars that do not appear along the main sequence
are often near the end of their lives. At the bottom centre of the
diagram are white dwarfs, such as the star Procyon B. They are
white because they are hot, but dim because they are small. White
dwarfs are cooling and will eventually become black. At the top
right of the diagram are red giants such as Aldebaran and
supergiants such as Betelgeuse and Antares. The outer layers of
these stars are cool and appear red, but they are bright because
they are so large. All of these giants will eventually explode.
Take It Further
Many different approaches have
been taken to graphically
displaying the data of the
Hertzsprung-Russell diagram.
Find at least three other versions
to the one shown here and
analyze how effective you think
they are. Begin your research at
ScienceSource.
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SKILLS YOU WILL USE
C12 Inquiry Activity
Skills Reference 2
Using a Star Chart
Your teacher will give you a simple star chart that can
be used for early evening observations.
Question
How is it possible to locate the positions of stars in the
night sky?
Observing and recording
observations
Communicating ideas,
procedures, and results in a
variety of forms
3. After you find the Big Dipper, locate the two stars
that make up the outside of the ladle. These are
know as the “pointer stars” because they point to
Polaris, the North Star. Follow the pointer stars
until you see a reasonably bright star. This is
Polaris. It is always in this position in the sky no
matter what the season or the time.
4. Follow along an arc until you reach a group of five
stars that forms a big W. (Depending on the time
of night and the season, this may look more like
an M.) This is the constellation Cassiopeia.
Materials & Equipment
• star chart
• flashlight with a red light (optional)
Procedure
1. While facing south, hold the chart over your
head, with the chart facing you. Read the chart
while looking up. Notice that east will be on your
left and west will be on your right. This should
match the labelling on the chart. A flashlight
casting a red light will allow you to read your
chart without having to let your eyes readjust to
see the stars.
2. Locate the Big Dipper. Identify it first on your star
chart. It is part of the constellation Ursa Major
and has the shape shown below. Then, find the
Big Dipper in the sky. You will see many more
stars in the sky than appear on the chart, but the
bright stars making up the Big Dipper should
stand
out.
5. Finally, go back to Polaris. It is part of the Little
Dipper, forming the last star in the handle. The
stars of the Little Dipper are not quite as bright as
the stars of the Big Dipper, but they are still easily
visible with the naked eye.
Analyzing and Interpreting
6. Describe how you used the Big Dipper to find
Polaris, Cassiopeia, and the Little Dipper.
7. If you were unable to find any of these stars or
groupings, explain what problems occurred that
prevented you from locating them in the sky.
8. The star Sirius is brighter than Polaris. Would it
make more sense to call Sirius the North Star,
instead of Polaris? Explain your answer.
Skill Practice
9. Using your star chart, identify three other
constellations in the northern sky.
Forming Conclusions
Cassiopeia
Little Dipper
(Ursa Minor)
Polaris
(North Star)
Big Dipper
(Ursa Major)
10. (a) If you were able to use the star chart effectively
in this activity, write one guideline to add to the
procedures that would help another student
using a star chart for the first time.
(b) If you were not able to use the star chart
effectively, list one or more questions that you
would need answered to help you find some
or all of the identified objects.
Figure 8.12 Three commonly observed constellations and Polaris
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SKILLS YOU WILL USE
C13 Design a Lab
Skills Reference 2
Planning for safe practices in
investigations
Drawing conclusions
Star Light, How Bright?
Just as stars vary in mass, colour, and temperature,
so they vary in brightness.
The brightness of stars viewed from Earth
depends on both their actual brightness (luminosity)
and their distance from Earth. If, for example, all stars
had the same brightness, then we could assume that
the ones that look dimmer to us from Earth are farther
away than those that look brighter. In a similar way,
identical flashlights held at different distances from us
will appear to vary in brightness, too.
There are many aspects to the relationship
between a light source’s actual brightness and its
distance from a viewer that can be explored. For
example, is it necessary to double the distance of a
light source from a viewer before the brightness of
the light source is cut in half? In this activity, you will
have an opportunity to investigate the brightnessdistance relationship by using flashlights or LED
penlights (but not laser sources of any kind) to
assess changes in brightness related to distance.
Question
How does the distance of a light source from an
observation point affect the apparent brightness of
the light source?
2. Determine how you will safely measure light
intensity from your various sources. For example:
Will the light be projected onto a screen and the
light intensity of the reflection observed, or will
one partner shine light into another partner’s eyes
from different distances? How will the intensities
of light be compared if they happen at different
times or if different people make the
observations? Can cameras be used? How will
you make your measurements?
3 . Design a procedure to carry out the investigation.
Include in it a list of materials and equipment you
will need. As well, design a data table to collect
information on your brightness observations.
4. Ask your teacher to check the design of your
procedure and data table.
5. Perform your investigation.
6. Prepare a formal lab report to document how you
conducted the investigation. At the end of the
report, summarize the results of your
investigation in one or more paragraphs.
7. Consider how you could refine your investigation
if you were to repeat it. Discuss your suggestions
with your teacher.
CAUTION: Laser pointers of any kind, including LED
laser pointers, are potentially damaging to the eyes and
are not appropriate for this experiment.
Design and Conduct Your Investigation
1. Select a variety of light sources to use in your
investigation. Possible light sources include nonlaser light sources of different brightness such as
penlight LEDs and flashlights with LED or
incandescent bulbs.
Figure 8.13 Possible set-up for activity
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C14 Quick Lab
Analyzing Stars by Their Spectral Patterns
Spectral patterns in stars are a little like star
“fingerprints.” By spreading a star’s light into its
spectral colours and “reading” the black spectral
lines that appear, we can identify the individual
chemical elements making up the star. Knowing
what elements are in a particular star gives us
information about how the star formed, whether it is
likely to be surrounded by rocky planets like Earth,
and how it will probably come to an end someday.
In this activity, you will analyze and compare
spectral patterns to determine the chemical make-up
of several stars.
Purpose
To identify the make-up of two mystery stars by
analyzing their spectral patterns
hydrogen
1. Looking at Figure 8.14, study the spectral
patterns for the five elements shown.
2. Answer the questions below, recording your
answers in your notebook.
Questions
3. Which three elements are visible iin mystery
star A?
4. Which three elements are visible in mystery
star B?
5. Which element listed in the spectral chart is not
present in either mystery star?
6. Make a sketch of the spectrum that would be
expected in a nebula that contains mainly
hydrogen and lithium.
H
helium
He
lithium
Li
beryllium
Be
magnesium
Mg
mystery star A
mystery star B
Figure 8.14 Spectral patterns for analysis
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Procedure
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CHECK and REFLECT
Key Concept Review
1. (a) What is a constellation?
(b) How many official constellations are
there?
2. What is a star called during its earliest stage
of formation?
3. What process must occur inside a forming
star before it can “switch on,” creating its
own light?
4. What main property of newly formed stars
determines how the star will evolve?
10. Using the Big Dipper as a point of
reference, explain how you would help
someone identify Polaris, the North Star, in
the night sky.
11. Organize the following list in correct order
of evolution.
(a) protostar
(b) nebula
(c) star
(d) red giant
12. Explain how the colour of a star is related
to its:
5. Most stars in the universe are what type?
(a) luminosity
6. Name the three characteristics by which
stars are plotted on the Hertzsprung-Russell
diagram.
(b) temperature
7. Explain the important concept about stars
that was revealed by the HertzsprungRussell diagram.
8. Use the Hertzsprung-Russell diagram in
Figure 8.11 on page 301 to answer the
following questions.
13. Sirius is orbited by a white dwarf known as
Sirius B. In the image below, Sirius B is the
tiny white dot to the lower left of Sirius.
Sirius B has a mass slightly less than the
Sun’s mass. What inference can you make
about the kind of star Sirius B will
eventually become?
(a) Which star’s surface temperature is
cooler, Antares or Vega?
(b) How many times more luminous is
Polaris than Procyon A?
(c) The Sun is of too low a mass to explode
in a supernova. As the Sun evolves and
slowly dies out, on which part of the
diagram would it be classified?
Connect Your Understanding
9. Describe how our view of constellations
and asterisms in the sky and on star charts
is misleading.
Question 13
Reflection
14. As you read in this section, newly formed
stars evolve in one of three main ways.
Think of a simple but creative method you
could use to summarize and remember the
stages in each of these life cycles.
For more questions, go to ScienceSource.
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